1. Field of the Invention
This invention relates to a method and apparatus for processing image signals in a color image reader and particularly to a method and apparatus for separating an image signal of a specific color from the image signal of its complementary color wherein a light signal from a color image is optically split into a light signal of the specific color and a light signal of the complementary color by means of a filter and these two light signals are photoelectrically converted and then compared with each other.
2. Description of the Prior Art
In general, a conventional color image reader is constructed so that a color signal from a color image is optically split into at least two light signals by means of a filter, a dichroic mirror or the like. Thereafter, these light signals are photoelectrically converted and predetermined processing is done on the image signals which were obtained by the above photoelectrical conversion, whereby the image signal of the specific color is separated, in other words, color separation is attained.
Color image signal processors for effecting such color separation have heretofore been of the following types:
(1) Image signals obtained by the above photoelectrical conversion are converted to binary values, i.e., digitized by comparing them with respective predetermined threshold values, and an image signal of a specified color is separated by executing logical operations on these binary-valued signals.
(2) Image signals obtained by the photoelectrical conversion are subjected to analogue operations to form analogue image signals and these analogue image signals are converted to binary values, i.e., digitized, by comparing them with respective threshold values, and an image signal of the specific color is separated by executing logical operations on these binary-valued signals.
In each of the above processors, image signals of a desired color are separated by comparing the image signals with certain threshold values. The comparison operation is disadvantageous in that it is difficult to obtain an image signal of a specific color whose luminance is low.
Furthermore, because of the variation in brightness of a light source for irradiating an original as well as the variation in sensitivity of the photoelectric conversion for each picture element, the image signals subjected to color separations are not uniform for all the picture elements so that the quality of the image obtained is deteriorated.
Accordingly, it is an object of the present invention to provide a method and apparatus for processing an image signal of simple construction wherein white balancing correction is effected, and at the same time, color separation is attained irrespective of the original's luminance, whereby a highly reliable image information signal is obtained.